Process for forming package-on-package structures

ABSTRACT

A device includes an inter-layer dielectric, a device die under the inter-layer dielectric; and a die-attach film under the inter-layer dielectric and over the device die, wherein the die-attach film is attached to the device die. A plurality of redistribution lines includes portions level with the die-attach film. A plurality of Z-interconnects is electronically coupled to the device die and the plurality of redistribution lines. A polymer-comprising material is under the inter-layer dielectric. The device die, the die-attach film, and the plurality of Z-interconnects are disposed in the polymer-comprising material.

PRIORITY CLAIM AND CROSS-REFERENCE

This application is a continuation of U.S. patent application Ser. No.14/638,839, entitled “Process for Forming Package-on-PackageStructures,” filed on Mar. 4, 2015, which application is a continuationof U.S. patent application Ser. No. 13/275,065, entitled “Process forForming Package-on-Package Structures,” filed on Oct. 17, 2011, whichapplication is incorporated herein by reference.

BACKGROUND

The fabrication of modern circuits typically involves several steps.Integrated circuits are first fabricated on a semiconductor wafer, whichcontains multiple duplicated semiconductor chips, each comprisingintegrated circuits. The semiconductor chips are then sawed from thewafer and packaged. The packaging processes have two main purposes: toprotect delicate semiconductor chips, and to connect interior integratedcircuits to exterior pins.

With the increasing demand for more functions, package-on-package (PoP)technology, in which two or more packages are bonded for expanding theintegration ability of the packages. With a high degree of integration,the electrical performance of the resulting PoP package is improved dueto the shortened connecting paths between components. By using the PoPtechnology, package design becomes more flexible and less complex.Time-to-market is also reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the embodiments, and the advantagesthereof, reference is now made to the following descriptions taken inconjunction with the accompanying drawings, in which:

FIGS. 1 through 38 are re cross-sectional views of intermediate stagesin the manufacturing of packages in accordance with various embodiments,wherein each of the packages include one or a plurality of device dies;and

FIGS. 39 through 41 are package-on-package structures comprising thepackages formed in the embodiments shown in FIGS. 1 through 38.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the embodiments of the disclosure are discussedin detail below. It should be appreciated, however, that the embodimentsprovide many applicable inventive concepts that can be embodied in awide variety of specific contexts. The specific embodiments discussedare merely illustrative, and do not limit the scope of the disclosure.

Packages and the methods of forming the same are provided in accordancewith various embodiments. The intermediate stages of forming thepackages are illustrated. The variations of the embodiments arediscussed. Throughout the various views and illustrative embodiments,like reference numbers are used to designate like elements.

FIGS. 1 through 9 illustrate the cross-sectional views of intermediatestages in the manufacturing of a package in accordance with embodiments.FIG. 1 illustrates carrier 20 and release layer 22 formed on carrier 20.Carrier 20 may be a glass carrier, a ceramic carrier, or the like.Release layer 22 may be formed of a polymer-based material, which may beremoved along with carrier 20 from the overlying structures, which willbe formed in subsequent steps. In an embodiment, release layer 22 isformed of an epoxy-based thermal-release material. In other embodiments,release layer may be formed of an ultra-violet (UV) glue. In someembodiments, release layer 22 is dispensed as a liquid and cured. Inalternative embodiments, release layer 22 is a laminate film, and islaminated onto carrier 20. The top surface of release layer 22 isleveled and has a high degree of co-planarity.

Dielectric layer 24 is formed on release layer 22, and redistributionlines (RDLs) 26 are formed. Dielectric layer 24 is alternativelyreferred to as inter-layer dielectric (ILD) 24 hereinafter. The bottomsurface of ILD 24 may be in contact with the top surface of releaselayer 22. In an embodiment, ILD 24 is formed of a photo-sensitivematerial such as polybenzoxazole (PBO), polyimide, benzocyclobutene(BCB), or the like, which may be easily patterned using a lithographymask. In alternative embodiments, ILD 24 may be formed of a nitride suchas silicon nitride, an oxide such as silicon oxide, phosphosilicateglass (PSG), borosilicate glass (BSG), boron-doped phosphosilicate glass(BPSG), or the like. RDLs 26 may include lower portions 26A whosebottoms contact release layer 22, and upper portions 26B whose bottomsurfaces contact top surface 24A of ILD 24. The lower portions 26A haveat least lower portion, and possibly all, in ILD 24, and may includenarrow portions and wider portions, wherein the wide portions may act asbond pads in subsequent bonding processes. In accordance with someexemplary embodiments, the formation of RDLs 26 and ILD 24 may includeforming ILD 24, etching and removing portions of ILD 24, forming anunder-bump-metallurgy (UBM, not shown) over ILD 24 and release layer 22,forming and patterning a photo resist (not shown) to cover portions ofthe UBM, and plating a metallic material to form RDLs 26. The photoresist and the exposed portions of the UBM are then removed. RDLs 26 maybe formed of copper, aluminum, tungsten, or the like.

FIG. 2 illustrates the formation of Z-interconnects 30, which may haveheight H (in the Z-direction) that is greater than the horizontaldimensions (W). Horizontal dimensions W may be measured in the X or Ydirections, which are parallel to major surface 20A of carrier 20.Z-interconnects 30 are electrically coupled to, and may be in physicalcontact with, RDLs 26. The longitudinal direction of Z-interconnects 30is in the Z direction, which is perpendicular to major surface 20A ofcarrier 20. In an embodiment, height H of Z-interconnects 30 is greaterthan about 50 μm, or greater than about 100 μm. The formation process ofZ-interconnects 30 may include forming a photo-sensitive material (notshown) over RDLs 26 and ILD 24, performing a lithography process to formopenings (not shown) in the photo-sensitive material, and plating ametallic material into the openings. After removing the photo-sensitivematerial, the remaining plated metallic material forms Z-interconnects30.

Referring to FIG. 3, die 34 is attached onto ILD 24, for example,through die-attach film 32. Die 34 may be a device die comprising activedevices such as transistors (not shown) therein. Die-attach film 32 maybe an adhesive film formed of an epoxy, silicon rubber, or the like.Although one die 34 is illustrated, a plurality of dies 34, which may beidentical to each other or different from each other, may be attachedonto ILD 24 through die-attach films. In an embodiment, die-attach film32 is dispensed in a liquid form. In alternative embodiments, die-attachfilm 32 is pre-attached onto the back surface of die 34, and is thenattached onto ILD 24. Die 34 includes electrical connectors 36 at thetop surface, wherein electrical connectors 36 may comprise solder balls,copper pillars, contact pads, and/or the like. Optionally, a pluralityof photo resist patterns 29 are formed, with each of photo resistpatterns 29 covering one of electrical connectors 36.

FIGS. 4A through 5B illustrate embodiments for embedding Z-interconnects30, die-attach film 32, and die 34 in polymer-comprising material 38. InFIG. 4A, polymer-comprising material 38 comprises a photo-sensitivematerial such as PBO, polyimide, BCB, or the like. Polymer-comprisingmaterial 38 may be applied in a liquid form, which is dispensed and thencured. Alternatively, polymer-comprising material 38 may be a laminatedfilm, and is laminated onto the structure shown in FIG. 3. The topsurface of polymer-comprising material 38 is higher than the topsurfaces of Z-interconnects 30. Next, as shown in FIG. 5A, thephoto-sensitive polymer-comprising material 38 is patterned through alithography process, so that electrical connectors 36 andZ-interconnects 30 are exposed through the openings inpolymer-comprising material 38.

FIGS. 4B and 5B illustrate alternative embodiments, whereinpolymer-comprising material 38 is not formed of a photo-sensitivematerial. Instead, polymer-comprising material 38 is formed of anon-photo-sensitive material such as a molding compound. Accordingly,referring to FIG. 4B, polymer-comprising material 38 may be formed onthe structure shown in FIG. 3 through compression molding, for example.In these embodiments, photo resist patterns 29 may be pre-formed (asshown in FIG. 3) over and overlapping electrical connectors 36 beforethe molding of polymer-comprising material 38. Next, as shown in FIG.5B, a grinding is performed, until photo resist patterns 29 andZ-interconnects 30 are exposed. Photo resist patterns 29 are thenremoved, so that electrical connectors 36 are exposed.

In the structure shown in FIG. 5A or 5B, polymer-comprising material 38acts as the substrate of the resulting package 50 (please refer to FIG.9), while Z-interconnects 30 acts as the through-substrate vias (TSVs,or through-vias) in the substrate. In the embodiments as shown in FIGS.3 through 5B, it is appreciated that the formation of Z-interconnects 30is performed using lithography processes, and the conventional methodsfor forming through-vias such as laser drilling is not used.Accordingly, the pitch of Z-interconnects 30 may be small.

In FIG. 6, an additional layer(s) of RDLs and ILDs are formed overpolymer-comprising material 38 and Z-interconnects 30. The additionalRDLs are electrically coupled to Z-interconnects 30. For example, in theillustrated embodiments, RDLs 40 are formed in ILD 42, and areelectrically coupled to Z-interconnects 30. The bottom layer of RDLs 40may be in physical contact with Z-interconnects 30. The material and theformation method of RDLs 40 may be essentially the same as that of RDLs26, although different materials and formation methods may be used.Also, the material and the formation method of ILD 42 may be selectedfrom the same available materials and formation methods of ILD 24,although different materials and formation methods may be used. The toplayer of ILD 42 may have openings, through which top surfaces of RDLs 40are exposed. The exposed portions of RDLs 40 may act as bond pads, andhence may be referred to as bond pads 40 hereinafter. The bond pads 40may further include an additional protective layer(s) (not shown) suchas a nickel layer, a palladium layer, a gold layer, or the like. In someembodiments, bond pads 40 are not formed in the region overlapping die34, as shown by dashed lines that represent additional ILD portions. Inalternative embodiments, bond pads 40 may be formed verticallyoverlapping die 34, and the illustrated dashed regions do not have ILD42 formed therein.

FIG. 7 illustrates the formation/mounting of electrical connectors 46.In some embodiments, electrical connectors 46 are solder balls, and areplaced on bond pads 40 and then reflowed. In alternative embodiments,electrical connectors 46 may have other structures including copperpillars, solder caps, palladium layers, gold layers, and/or the like. Insome embodiments, no electrical connectors are formed overlapping die34, and the dashed solder balls 46 are not formed. In alternativeembodiments, as shown by dashed electrical connectors 46, electricalconnectors 46 may be formed vertically overlapping die 34.

FIG. 8 illustrates the demounting of carrier 20, which may be achievedby removing release layer 22. In the exemplary embodiment whereinrelease layer 22 is formed of a UV glue, a UV light may be projectedonto release layer 22, so that release layer 22 and carrier 20 may beremoved from ILD 24 and RDLs 26.

In FIG. 9, the wafer as shown in FIG. 8 is singulated into a pluralityof packages 50, for example, along scribe lines 48. In the resultingpackage 50 as in FIG. 9, RDL portions 26A have top surfaces 26C levelwith, and forming a planar surface with, top surface 24A of ILD 24.Furthermore, RDL portions 26B are at the same level as portions ofdie-attach film 32, wherein die-attach film 32 and die 34 are disposedunder top surface 38A of polymer-comprising material 38. Alternativelystating, die-attach film 32, die 34, and Z-interconnects 30 are embeddedin polymer-comprising material 38. Package 50 may be a fan-out package,wherein RDLs 40 extends to a greater area than die 34, so that the formfactor of package 50 is small.

FIGS. 10 through 26 illustrate cross-sectional views of intermediatestages in the formation of a package in accordance with alternativeembodiments. Unless specified otherwise, the materials and formationmethods of the components in these embodiments are essentially the sameas the like components, which are denoted by like reference numerals inthe embodiment shown in FIGS. 1 through 9. The formation details of theembodiment shown in FIGS. 10 through 26 (and the subsequent embodimentsin FIGS. 27 through 38) may thus be found in the discussion of theembodiments shown in FIGS. 1 through 9. In the embodiments shown inFIGS. 10 through 26, any two features with reference numerals offset by100 compared to the reference numerals in FIGS. 1 through 9 may indicatethat they are essentially the same type of features, formed of similarmaterials, and/or using the same methods, except these two features arein different levels of the resulting package.

The initial steps of these embodiments are shown in FIGS. 10 through14B, and are essentially the same as shown in FIGS. 1 through 5B.Accordingly, the details are not repeated herein. In FIGS. 13A and 14A,polymer-comprising material 38 comprises a photo-sensitive material,while in FIGS. 13B and 14B, polymer-comprising material 38 may be formedof a non-photo-sensitive material. The respective available materialsand formation methods are disclosed in the embodiments shown in FIGS. 4Athrough 5B. In FIGS. 14A and 14B, openings 125 may be formed in thephoto-sensitive polymer-comprising material 38, so that Z-interconnects30 are exposed. Next, as shown in FIG. 15, RDLs 126 are formed toelectrically connect to Z-interconnects 30 and electrical connectors 36of die 34, followed by the formation of Z-interconnects 130, whereinZ-interconnects 130 and RDLs 126 may be formed using essentially thesame methods as forming Z-interconnects 30 and RDLs 26, respectively.

Next, as shown in FIG. 16, die 134 is attached to RDLs 126 throughdie-attach film 132. Die 134 may be a device die or another type ofpackage component, and electrical connectors 136 of die 134 are exposed.In FIG. 17A, photo-sensitive polymer-comprising material 138 is formedover die 134 and Z-interconnects 130, and is then patterned using alithograph process. The resulting structure is shown in FIG. 18A,wherein electrical connectors 136 and Z-interconnects 130 are exposedthrough the openings in polymer-comprising material 138. FIGS. 17B and18B illustrate a process similar to what is shown in FIGS. 17A and 18A,except that polymer-comprising material 138 is a non-photo-sensitivematerial. Accordingly, as in FIG. 17B, photo resist patterns 129 may beformed to cover electrical connectors 136, and are then removed toexpose the underlying electrical connectors 136.

FIGS. 19 through 22B illustrate the stacking of and additional level,which process includes the formation of RDLs 226 and Z-interconnects 230(FIG. 19), the attachment of die 234 to RDLs 226 through die-attach film232 (FIG. 20), and the application and the patterning of photo-sensitivepolymer-comprising material 238 (FIGS. 21A and 22A), or the applicationand the patterning of non-photo-sensitive polymer-comprising material238 (FIGS. 21B and 22B). The process details and the respectivematerials may be referred to in the description of FIGS. 2 through 5B,and are not repeated herein.

In FIG. 23, RDLs 40 and ILD 42 are formed. Electrical connectors 46 arethen formed, and the resulting structure is shown in FIG. 24. Next, asshown in FIG. 25, release layer 22 and carrier 20 are removed from overILD 24 and RDLs 26. The resulting wafer as shown in FIG. 25 is thensingulated along scribe lines 48 to form a plurality of packages.

In the resulting package 150 as in FIG. 26, RDL portions 26A have topsurfaces level with top surface 24A of ILD 24. Furthermore, RDL portions26B are at the same level as portions of die-attach film 32. Dies 34,134, 234 are stacked and are in a plurality of levels of package 150.The plurality of levels of RDLs 26, 126, and 226 is interconnected toeach other through Z-interconnects 30, 130, and 230, and forms fan-outconnections for dies 34, 134, 234. Dies 34, 134, and 234 and therespective die-attach films 32, 132, and 232 are embedded in therespective polymer-comprising materials 38, 138, and 238, in which thecorresponding Z-interconnects 30, 130, 230 are located.

In the embodiments shown in FIGS. 1 through 26, dies are embedded insidethe respective packages. FIGS. 27 through 38 illustrate alternativeembodiments, wherein in addition to the embedded dies, dies may furtherbe attached on the surfaces of packages. Referring to FIG. 27, ILD 24and RDLs 26 are formed on release layer 22, which is further located oncarrier 20. RDLs 26 includes traces 26A1 and 26A2, which are the tracesfor forming bump-on-trace joints with die 70 (not shown in FIG. 27,please refer to FIG. 38). Next, as shown in FIG. 28, Z-interconnects 30are formed on RDLs 26. In FIG. 29, die 34 is attached to RDLs 26A1 and26A2 through die-attach film 32, and optional photo resist patterns 29may be formed to cover electrical connectors 36 of die 34. In theseembodiments, die-attach film 32 may have a bottom surface in contactwith the top surface of release layer 22.

FIGS. 30A and 31A illustrate the forming/dispensing and the patterningof photo-sensitive polymer-comprising material 38. Alternatively, asshown in FIGS. 30B and 31B, non-photo-sensitive polymer-comprisingmaterial 38 are applied/molded and patterned. Next, as shown in FIG. 32,ILD 42 and RDLs 40 are formed, and are electrically coupled toZ-interconnects 30 and electrical connectors 36. RDLs 40 may includetraces 40A1 and 40A2. Release layer 22 and carrier 20 may then bedemounted from ILD 24 and RDLs 26, as shown in FIG. 33. A singulationmay then be performed along scribe lines 48 to separate the respectivewafer into individual packages.

In FIG. 34, die 60 is bonded to RDLs 26 of a resulting package, forexample, through a bump-on-trace structure. The respective electricalconnectors 62 of die 60 may include metal pillars (which may be formedof copper or a copper alloy), and solder regions 64 that are in contactwith the top surfaces and the sidewalls of RDL portions/traces 26A1 and26A2. RDL portions 26A1 and 26A2 may be thin traces instead of largebond pads, although large bond pads may also be used. Next, as shown inFIG. 35, underfill 66 is dispensed into the gap between die 60 and RDLs26A1/26A2 and die-attach film 32. Underfill 66 is then cured.

Referring to FIG. 36, electrical connectors 46, which may be solderballs, for example, are mounted/formed on RDLs 40. In FIG. 37, anadditional die 70 is further bonded to RDLs 40, for example, through thebump-on-trace structure. The respective electrical connectors 72 of die70 may include metal pillars (which may be formed of copper or a copperalloy) and solder regions 74 that are in contact with the top surfacesand the sidewalls of RDLs 40A1 and 40A2. RDLs 40A1 and 40A2 may be thintraces instead of large bond pads. Next, as shown in FIG. 38, underfill76 is dispensed into the gap between die 70 and RDLs 40 and cured.Please note that FIG. 38 shows a structure flipped from what is shown inFIG. 37.

In the resulting package 250 as in FIG. 38, RDL portions 26A have topsurfaces level with top surface 24A of ILD 24. Furthermore, RDL portions26B may be at the same level as portions of die-attach film 32. Dies 60and 70 are bonded on the opposite surfaces of package 250. The pluralityof levels of RDLs is interconnected to each other throughZ-interconnects. Furthermore, die 34 and die-attach film 32 are embeddedin polymer-comprising materials 38, in which Z-interconnects 30 arelocated. Package 250 is also a fan-out package.

The packages as shown in FIGS. 9, 26, and 38 may be stacked on eachother in any combination to form package-on-package structures. Forexample, FIG. 39 illustrates an embodiment in which two of the packages50 are stacked. In this structure, dies 34 in two packages 50 may havean identical structure or have different structures. FIG. 40 illustratesan embodiment in which package 250 as in FIG. 38 is stacked on package50 as in FIG. 9. FIG. 41 illustrates an embodiment in which packages 150as in FIG. 26 is stacked on package 50 as in FIG. 9. It is noted thatthere are more combinations, which are also in the scope of theembodiments.

In accordance with embodiments, a device includes an inter-layerdielectric, a device die under the inter-layer dielectric; and adie-attach film under the inter-layer dielectric and over the devicedie, wherein the die-attach film is attached to the device die. Aplurality of redistribution lines includes portions level with thedie-attach film. A plurality of Z-interconnects is electronicallycoupled to the device die and the plurality of redistribution lines. Apolymer-comprising material is under the inter-layer dielectric. Thedevice die, the die-attach film, and the plurality of Z-interconnectsare disposed in the polymer-comprising material.

In accordance with other embodiments, a device includes an inter-layerdielectric, a device die under the inter-layer dielectric, and adie-attach film attaching the device die to the inter-layer dielectric,wherein a top surface of the die-attach film contacts a bottom surfaceof the inter-layer dielectric. A first plurality of redistribution linesincludes first portions level with the die-attach film, and secondportions in the inter-layer dielectric. A plurality of Z-interconnectsis disposed under the first plurality of redistribution lines, andelectronically coupled to the device die and the first plurality ofredistribution lines. A polymer-comprising material is under theinter-layer dielectric, wherein the device die, the die-attach film, andthe plurality of Z-interconnects are disposed in the polymer-comprisingmaterial. A second plurality of redistribution lines is under theplurality of Z-interconnects, wherein the first and the second pluralityof redistribution lines are on opposite sides of, and are electricallycoupled through, the plurality of Z-interconnects.

In accordance with yet other embodiments, a device an inter-layerdielectric, a first device die under the inter-layer dielectric, adie-attach film over and attached to the device die, and a firstplurality of redistribution lines comprising first portions level with aportion of the die-attach film, and second portions in the inter-layerdielectric and contacting the die-attach film. A plurality ofZ-interconnects is disposed under the first plurality of redistributionlines and electronically coupled to the first device die and the firstplurality of redistribution lines. A polymer-comprising material isdisposed under the inter-layer dielectric, wherein the first device die,the die-attach film, and the plurality of Z-interconnects are disposedin the polymer-comprising material. A second plurality of redistributionlines is under the plurality of Z-interconnects, wherein the first andthe second plurality of redistribution lines are on opposite sides of,and are electrically coupled through, the plurality of Z-interconnects.A second device die is over and bonded to the second portions of thefirst plurality of redistribution lines. A third device die is under andbonded to the second plurality of redistribution lines, wherein thesecond and the third device dies are at opposite surfaces of the device.

In accordance with yet other embodiments, a method includes forming aninter-layer dielectric over and in contact with a release layer, whereinthe release layer is further disposed over a carrier, forming a firstplurality of redistribution lines, wherein the first plurality ofredistribution lines comprises first portions extending into theinter-layer dielectric and in contact with the release layer, and secondportions over the inter-layer dielectric, attaching a device die to atop surface of the inter-layer dielectric or top surfaces of the firstportions of the first plurality of redistribution lines through adie-attach film, and forming Z-interconnects over and electricallycoupled to the first plurality of redistribution lines. After the stepof forming the Z-interconnects, forming a polymer-comprising material,wherein the second portions of the first plurality of redistributionlines, the die-attach film, and the device die are in thepolymer-comprising material. A second plurality of redistribution linesis formed over the polymer-comprising material, wherein the secondplurality of redistribution lines is electrically coupled to the devicedie, and electrically coupled to the first plurality of redistributionlines through the Z-interconnects.

Although the embodiments and their advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the embodiments as defined by the appended claims. Moreover,the scope of the present application is not intended to be limited tothe particular embodiments of the process, machine, manufacture, andcomposition of matter, means, methods and steps described in thespecification. As one of ordinary skill in the art will readilyappreciate from the disclosure, processes, machines, manufacture,compositions of matter, means, methods, or steps, presently existing orlater to be developed, that perform substantially the same function orachieve substantially the same result as the corresponding embodimentsdescribed herein may be utilized according to the disclosure.Accordingly, the appended claims are intended to include within theirscope such processes, machines, manufacture, compositions of matter,means, methods, or steps. In addition, each claim constitutes a separateembodiment, and the combination of various claims and embodiments arewithin the scope of the disclosure.

What is claimed is:
 1. A method comprising: forming a first plurality ofredistribution lines; forming first Z-interconnects over andelectrically coupled to the first plurality of redistribution lines;attaching a first device die, wherein a portion of the first device dieis substantially coplanar with a portion of the first Z-interconnects;after the forming the first Z-interconnects, encapsulating the firstdevice die and the first Z-interconnects in a first encapsulationmaterial; and forming a second plurality of redistribution lineselectrically coupling to the first device die, wherein the firstplurality of redistribution lines and the second plurality ofredistribution lines are on opposite sides of the first Z-interconnects.2. The method of claim 1, wherein the first encapsulation materialcomprises a photo-sensitive material, and the method further comprises,after the encapsulating the first encapsulation material, performing alithography process on the first encapsulation material to expose thefirst Z-interconnects and electrical connectors of the first device die.3. The method of claim 1, wherein the first encapsulation materialcomprises a molding compound covering photo resist patterns of the firstdevice die, with the photo resist patterns covering electricalconnectors of the first device die, and wherein the method furthercomprises: grinding the first encapsulation material to expose the photoresist patterns and the first Z-interconnects; and removing the photoresist patterns to reveal the electrical connectors.
 4. The method ofclaim 1 further comprising: stacking a second device die over the secondplurality of redistribution lines; forming second Z-interconnects overand electrically coupled to the second plurality of redistributionlines; and molding the second device die and the second Z-interconnectsin a second encapsulation material.
 5. The method of claim 1, whereinthe first plurality of redistribution lines is formed over a releaselayer, and the method further comprises: detaching the release layerfrom the first plurality of redistribution lines; and bonding a seconddevice die onto the first plurality of redistribution lines.
 6. Themethod of claim 5, wherein the second device die is bonded onto thefirst plurality of redistribution lines through bump-on-tracing bonding.7. The method of claim 1, wherein the first plurality of redistributionlines comprises first portions extending into an inter-layer dielectric,and second portions over the inter-layer dielectric.
 8. A methodcomprising: forming a first plurality of redistribution lines; formingZ-interconnects over and electrically coupled to the first plurality ofredistribution lines; placing a first device die, wherein a portion ofthe first device die is substantially coplanar with a portion of theZ-interconnects; encapsulating the Z-interconnects and the first devicedie in a photo-sensitive material, with the photo-sensitive materialcomprising portions covering electrical connectors of the first devicedie and the Z-interconnects; removing the portions of thephoto-sensitive material, wherein the electrical connectors of the firstdevice die and the Z-interconnects are exposed through openings in thephoto-sensitive material; and forming a second plurality ofredistribution lines over the photo-sensitive material and electricallycoupling to the electrical connectors of the first device die and theZ-interconnects.
 9. The method of claim 8 further comprising: forming apatterned dielectric layer over a carrier, wherein the first pluralityof redistribution lines comprises first portions extending into thepatterned dielectric layer, and second portions over the patterneddielectric layer, and wherein the first device die overlaps thepatterned dielectric layer.
 10. The method of claim 8, wherein the firstdevice die overlaps the first plurality of redistribution lines.
 11. Themethod of claim 10 further comprising bonding a second device die tophysically contact the first plurality of redistribution lines.
 12. Themethod of claim 8, wherein the second plurality of redistribution linesextends into the openings to contact the electrical connectors of thefirst device die and the Z-interconnects.
 13. The method of claim 8further comprising attaching a second device die over the secondplurality of redistribution lines.
 14. The method of claim 8, whereinthe first plurality of redistribution lines is in contact with a releaselayer over a carrier, and the method further comprises removing therelease layer to expose the first plurality of redistribution lines. 15.A method comprising: forming a patterned dielectric layer over acarrier; forming a first plurality of redistribution lines comprisingfirst portions extending into the patterned dielectric layer, and secondportions over the patterned dielectric layer; forming Z-interconnectsover and electrically coupled to the first plurality of redistributionlines; placing a first device die over the patterned dielectric layer,wherein the first device die comprises a plurality of photo-sensitiveregions covering electrical connectors of the first device die;encapsulating the Z-interconnects and the first device die in anencapsulation material, with the encapsulation material comprisingportions covering the plurality of photo-sensitive regions and theZ-interconnects; and removing the plurality of photo-sensitive regionsto reveal the electrical connectors.
 16. The method of claim 15 furthercomprising: before the removing the plurality of photo-sensitiveregions, planarizing the encapsulation material to expose the pluralityof photo-sensitive regions.
 17. The method of claim 15 furthercomprising forming a second plurality of redistribution lines over theencapsulation material and electrically coupling to the electricalconnectors of the first device die and the Z-interconnects.
 18. Themethod of claim 17, wherein the second plurality of redistribution linesextends into openings formed by the removed plurality of photo-sensitiveregions to contact the electrical connectors of the first device die.19. The method of claim 15 further comprising bonding a second devicedie directly to the first plurality of redistribution lines.
 20. Themethod of claim 15, wherein the plurality of photo-sensitive regions isa discrete region, and wherein the encapsulation material is filled intospaces between the plurality of photo-sensitive regions.